1. Field of the Invention
The present invention relates to a method of and system for controlling fuel injection for a direct injection-spark ignition type of internal combustion engine which is supplied with fuel directly into a combustion chamber through a fuel injector, and, more particularly, to a fuel injection control system in which learning control is performed to learn quantitative variations of fuel injection due to individual differences of fuel injectors.
2. Description of the Related Art
Typically, fuel injection control systems for general gasoline engines control an air-fuel ratio of air-fuel mixture by performing quantitative regulation of fuel injection and intake air according to engine operating conditions. In order to avoid aggravation of controllability of fuel injection due to various factors such as individual differences of fuel injectors and changes in engine operation surroundings, it is popular to perform feedback control of the amount of fuel injection on an output signal provided by an oxygen (O2) sensor disposed in an exhaust passage of the engine. In the fuel injection control, learning quantitative variations in fuel injection from the output signal of the oxygen sensor and reflecting the result in basic fuel injection control is effective to improve transient responsiveness of air-fuel ratio control and air-fuel ratio control accuracy while the feedback control of air-fuel ratio is not implemented.
Because, in a direct injection-spark ignition type of internal combustion engine which is supplied with fuel directly into a combustion chamber under high pressure, fuel is sprayed at a pressure remarkably higher as compared with port injection, quantitative variations in fuel injection are apt to be large as a logical consequence. Furthermore, an injector for the direct injection-spark ignition type of internal combustion engine has to have a relatively large nozzle, which is one of causes of large quantitative variations, In particular, a micro-flow characteristic of the fuel injector is irregular in a period of engine idling in which a time for which the injector remains open is very short differently from a period other than the idling period in which the micro-flow characteristic is linear (see FIG. 7). The micro-flow characteristics are significantly different due to individual differences of injectors. That is to say, since the injector for the direct injection-spark ignition type of internal combustion engine has the property of causing quantitative variations in fail injection while spraying a small amount of fuel, it is a reality that the direct ignition-spark ignition type of internal combustion engine has a strong demand for learning control of fuel injection for actual quantitative variations in fuel injection. However, the direct ignition-spark ignition type of internal combustion engine is usually operated in a stratified charge combustion state in an engine operating region of lower engine loads. In the stratified combustion state, a mean air-fuel ratio in a combustion chamber (which is hereafter referred to as a mean combustion chamber air-fuel ratio) is remarkably high, in other words, on a remarkably lean side, so that the oxygen sensor is hard to detect an air-fuel ratio with high precision as conventional. In consequence, although quantitative variations in fuel injection are apt to become large during idling in the lower load and stratified charge combustion region in which the engine is operate so often, it is difficult to perform the learning control of quantitative variations in fuel injection and the air-fuel ratio control accurately in that region.
In this regard, a fuel injection control system for a direct ignition-spark ignition type of internal combustion engine, such as disclosed in, for example, Japanese Unexamined Patent Publication No. 5- 99051, performs learning control of a deviation of an actual quantity of fuel injection from a target quantity of fuel injection, i.e. a quantitative variation in fuel injection, on the basis of a measurement of quantitative fuel consumption during a predetermined number of times of fuel injection while the engine is idling. In the fuel injection control, different values are employed as flow rate conversion coefficient Kps and Kpb for a regular flow characteristic of the fuel injector which is used in a proportional region where the quantity of fuel injection is proportional to a period of time for which the fuel injector is kept open (duration of injector opening) and a micro-flow characteristic of the fuel injector which is used in a non-proportional region where the quantity of fuel injection is not proportional to a period of time for which the fuel injector is kept open (duration of injector opening), respectively. For an intermediate region between the proportional and non-proportional regions, a conversion coefficient is gained by linear approximate calculation with use of the conversion coefficient Kpb and Kps.
The prior art fuel injection control system described above defines a micro-flow characteristic for the non-proportional region by a single flow rate conversion coefficient, the control of fuel injection can not be so precise in the non-proportional region. Specifically, as shown in FIG. 7 by way of example, when an injection pulse width Ti, which is a measurement of how long the fuel injector is kept open, is smaller than a specified injection pulse width Ti*, the quantity of fuel injection by the fuel injector is not proportional to the injection pulse width Ti and irregularly changes with respect to a change in injection pulse width Ti. Therefore, in the case where the micro-flow characteristic, i.e. the relationship between a quantity of fuel injection and an injection pulse width is defined by a single conversion efficiency Kps, the control of fuel injection is not precise at all in the non-proportional region. In consequence, though the prior art fuel injection control system is adapted to correct the conversion efficiency Kps by learning quantitative variations of fuel injection through a fuel injector, it can not be said that the fuel injection control is precise during engine idling where a quantity of fuel injection is small and, therefore, the prior art fuel injection control system leaves room for further improvement in regard to emission control and fuel consumption.
It is therefore an object of the present invention to provide a fuel injection control system capable of learning quantitative variations of fuel injection with high precision in a region of narrow injection pulse widths which is realized through a fully worked-out control sequence.
The foregoing object of the present invention is accomplished by a fuel injection control system which, while feedback controlling a quantity of fuel injection so as to provide a constant idling engine speed during idling, performs learning quantitative variations in fuel injection on the basis of a feedback control value for various duration of injector opening by forcibly changing a quantity of fuel injection, necessary to keep the constant idling engine speed, for a plurality of specified fuel injection timings which take place in turn.
Specifically, as shown in FIG. 1, the fuel injection control system, that is incorporated with a direct injection-spark ignition type of internal combustion engine equipped with a fuel injector 12 for spraying fuel directly into a combustion chamber 6 of the engine 1 in a compression stroke of each cylinder 2 so as to cause stratified charge combustion in a specified engine operating region of lower engine loads and lower engine speeds defined for stratified charge combustion, comprises intake air quantity regulation means 220 for regulating a quantity of intake air that is admitted into the combustion chamber 6, learning control means 52 learning a quantitative variation of an actual quantity of fuel injection from a target quantity of fuel injection while the engine idles in the specified engine operating region for stratified charge combustion, intake air flow control means 45 for controlling the intake air regulation means 220 so as to provide a constant quantity of intake air that is admitted into the combustion chamber while learning the quantitative variation, and fuel injection quantity control means 51 for feedback controlling the actual quantity of fuel injection so as to bring an engine speed ne into a specified idling engine speed while learning the quantitative variation, and ignition timing control means 50 for adopting a plurality of specified fuel injection timings in turn while learning the quantitative variation of fuel injection. The learning control means 52 implements the learning of a quantitative variation of an actual quantity of fuel injection from a target quantity of fuel injection on the basis of a feedback control value for each specified fuel injection timing.
According to the fuel injection control system, while the engine 1 idles in the specified engine operating region for stratified charge combustion, the-intake air flow control means 45 controls the intake air regulation means 220 so as to provide a constant quantity of intake air that is admitted into the combustion chamber and the fuel injection quantity control means 51 feedback controls the actual quantity of fuel injection so as to bring an engine speed ne into a specified idling engine speed. The learning control means 52 learns a quantitative variation of an actual quantity of fuel injection from a target quantity of fuel injection on the basis of a feedback control value. In consequence, the fuel injection control system performs direct learning of a quantitative variation of fuel injection during engine operation in a stratified charge combustion mode as well as during an ordinary engine idling mode. Furthermore, while learning a quantitative variation of fuel injection during engine operation in the stratified charge combustion mode, the duration o injector opening is varied so as to bring an engine speed ne into a specified idling engine speed according to the specified ignition timings that take place in turn. Since the learning control is implemented at every specified ignition timing, quantitative variations of fuel injection are obtained for different duration of injector opening.
Accordingly, even when a quantity of fuel injection does not vary in proportional to a change in duration of injector opening while the fuel injector is kept open for an extremely short period of time, it is enabled to learn the delicate relationship between duration of injector opening and an actual quantity of fuel injection, which results in having an accurate grasp of the characteristic of fuel injection of the fuel injector. This makes it possible to eliminate a quantitative variation of fuel injection even while the engine idles by determining a quantity of fuel injection on the basis of the characteristic of fuel injection of the fuel injector with an effect of significantly lowering emission levels and improving fuel consumption.
The fuel injection control system may calculates the target quantity of fuel injection according at least to an engine operating condition and controls the fuel injector to keep open for a period of time necessary for the target quantity of fuel injection according to the characteristic of fuel injection. In this instance, the characteristic of fuel injection is corrected on the basis of the learned values for the specified fuel injection timings.
The fuel injection control system may control the intake air quantity regulation means to admit intake air into the combustion chamber so as to provide a mean excess air ratio xcex3 equal to or greater than 1.3 while learning quantitative variations of fuel injection. Since a change in engine output relative to a change in fuel injection quantity becomes larger in a lean state where the excess air ratio xcex3 is equal to or greater than 1.3, the learning control of quantitative variations of fuel injection is performed with a high sensitivity. Moreover, since a change in engine output relative to a change in ignition timing becomes larger in a the lean state, it is enabled to learn quantitative variations of fuel injection over a relatively wide range of duration of injector opening even through the ignition timing is not varied so significantly. This makes it precise to learn the characteristic of fuel injection of the fuel injector.
The fuel injection control system may further calculate a charging efficiency of intake air admitted into the combustion chamber and corrects the learned value of a quantity of fuel injection on the basis of the charging efficiency of intake air. Although the accuracy of the learning control of quantitative variations of fuel injection on the basis of a learned value of a quantity of fuel injection possibly lowers due to fluctuations in engine output which occur due to a change in the charging efficiency of intake air, however, correcting the learned value of a quantity of fuel injection that is made on the basis of an actual charging efficiency of intake air yields improvement of learning accuracy.
The fuel injection control system may be configured so as to force appliances such as, for example, a compressor of an air conditioning system as an external engine load to turn off. Although the accuracy of the learning control of quantitative variations of fuel injection on the basis of a learned value of a quantity of fuel injection possibly lowers due to a change in fuel injection quantity which occurs due to a change in engine load while learning a quantitative variation of fuel injection of the fuel injector, however, forcibly turning off the appliances as an external engine load while learning a quantitative variation of fuel injection of the fuel injector yields improvement of learning accuracy.
The fuel injection control system may learn quantitative variations of fuel injection of the fuel injector after the engine warms up. There is a general tendency for the quantity of fuel injection necessary to keep an idling speed to slightly increase due to insufficient atomization and evaporation of fuel until the engine warms up. This tendency is eased as the temperature of engine raises. For this reason, besides the accuracy of the learning control of quantitative variations of fuel injection on the basis of a learned value of a quantity of fuel injection is low until the engine warms up, the learned result changes with time. In this regard, the fuel injection control system performs the learning control of quantitative variations of fuel injection after the engine warms up, so as thereby to provide the learning control of quantitative variations of fuel injection with a sufficiently high accuracy.
The fuel injection control system may further perform the learning control of quantitative variations when the engine cooling water is higher than a specified temperature even before the engine warms up as well as after the engine warms up. This increases the frequency in learning the quantitative variations in fuel injection, so as to realize high precision of the learning of quantitative variations in fuel injection. In this instance, as long as the engine is in a semi-warmed condition, a learned value is slightly inaccurate as compared with one gained after the engine warms up, but it is reliable.
The fuel injection control system may correct a learned value according to a temperature of the engine cooling water in consideration of a tendency for the quantity of fuel injection necessary to keep an idling speed to increase with a raise in the temperature of engine cooling water. This makes it possible to estimate a fuel injection characteristic for after warming up on the basis of a learned result gained during a semi-warmed state of the engine, which realizes high precision of the learning of quantitative variations in fuel injection.
The fuel injection control system is suitably incorporated in a multiple cylinder engine and, in this case, learns a quantitative variation on the basis of a mean value of the feedback control values in a specified combustion cycles by cylinder. Since this makes it possible to precisely learn a quantitative variation of fuel injection of the fuel injector for each cylinder, fuel injection for the entire engine can be controlled with high precision by correcting fuel injection characteristics of the respective fuel injections on the basis of learned results, respectively.